Fixed PinchViewport getting resized to empty size

Fixed PinchViewport getting resized to empty size.

Looks like we forgot to save the visible viewport to the local member
in RenderWidget. This was a problem when DisableAutoResize was called
(such as by the BrowserPluginGuest, as in the bug) since it would
pass the member variable along, resetting the pinch viewport to 0,0.
Since the pinch viewport was recently made to clip its contents, this
resulted in a blank screen.

BUG=371061
Committed: https://src.chromium.org/viewvc/chrome?view=rev&revision=270331

[bokan, 2014-05-13 16:26:55]

PTAL

[kevers, 2014-05-13 17:37:57]

lgtm

[jam, 2014-05-13 21:06:07]

lgtm

[bokan, 2014-05-13 22:22:14]

The CQ bit was checked by bokan@chromium.org

[commit-bot: I haz the power, 2014-05-13 22:22:51]

CQ is trying da patch. Follow status at
 https://chromium-status.appspot.com/cq/bokan@chromium.org/284883002/20001

[commit-bot: I haz the power, 2014-05-14 02:53:11]

FYI, CQ is re-trying this CL (attempt #1).
Please consider checking whether the failures are real,
and report flakes to chrome-troopers@google.com.
The failing builders are:
  win_chromium_rel on tryserver.chromium
(http://build.chromium.org/p/tryserver.chromium/builders/win_chromium_rel/buil...)

[commit-bot: I haz the power, 2014-05-14 04:20:30]

Change committed as 270331